The present invention relates to a cathode ray tube, and in particular, to a cathode ray tube for use in a projection type image display apparatus such as a projection type TV receiver and a video projector.
The projection type image display apparatus employs three projection type cathode ray tubes for emitting red, green and blue lights, respectively. Three images on the respective panel portions of the three projection type cathode ray tubes are enlarged by a projection lens and are combined on a screen. In a projection type image display apparatus, since images on a panel portion of 127 mm (5 inches) to 178 mm (7 inches) in diagonal dimension of a projection type cathode ray tube are enlarged and projected onto a screen of 1,016 mm (40 inches), for example, in diagonal dimension, the images formed on the panel portion of the projection type cathode ray tube are required to be highly bright and good in focus characteristics. That is to say, it is necessary to limit degradation in focus characteristics to acceptable amounts even when a beam current is increased to produce a highly bright image on the panel portion.
In the present, a horizontal deflection frequency in the projection type image display apparatus is changed from 15 kHz for the conventional NTSC signals to 30 kHz for the Hivision (the Japanese high-definition television format) signals, and the use of signals capable of higher-resolution display has become the most common in the projection type image display apparatus. Consequently, there is a demand for improvement on focus characteristics of projection type cathode ray tubes so that the projection type image display apparatus can produce higher-resolution images. Focus characteristics can be improved by increasing a diameter of a neck portion of a projection type cathode ray tube from 29 mm to 36 mm, and thereby increasing a diameter of a main lens of an electron gun of the projection type cathode ray tube, but, in view of use in place of conventional projection type cathode ray tubes, it is demanded that focus characteristics should be improved by using projection type cathode ray tubes having a 29 mm-diameter neck portion.